The present invention generally relates to electrical components and more particularly, to a thin film light emitting element including a light emitting layer which effects electroluminescence in response to the application of an electric field thereto. The light emitting element is made of a compound semiconductor material, e.g. ZnS, etc. as its base material and an activator material such that the compound semiconductor material and the activator material are set at a stoichiometric composition in order to remarkably stabilize light emitting characteristics of the light emitting layer.
Conventionally, there have been proposed thin film light emitting elements having a structure of two insulating films, in which a light emitting layer made of a compound semiconductor m.aterial such as ZnS, ZnSe, etc. is interposed between first and second dielectric layers. In the known thin film light emitting elements, an activator such as various rare earth elements, transition metals, etc. is doped in the compound semiconductor material in order to secure high dielectric strength, luminous efficiency and operational stability when the known thin film light emitting elements are subjected to a high AC drive voltage of about 10.sup.6 V/cm. Especially, ZnS:Mn type thin film light emitting elements each including a light emitting layer made of zinc sulfide (ZnS) as its base material and manganese (Mn) as its activator are put to practical use for matrix type planar display units of information processing apparatuses, television sets, etc. and a fundamental structure of the prior art ZnS:Mn type thin film light emitting elements is shown in FIG. 1. Each of the prior art ZnS:Mn type thin film light emitting elements include a glass substrate 1, a transparent electrode 2 made of In.sub.2 O.sub.3, SnO.sub.2, etc., a first dielectric layer 3, a light emitting layer 4, a second dielectric layer 5 and a back electrode 6 made of Al which are stacked one on another in this order. The first dielectric layer 3 is of a single-layer film or a multi-layer film formed by sputtering or electron beam evaporation of Y.sub.2 O.sub.3, Ta.sub.2 O.sub.5, TiO.sub.2, Al.sub.2 O.sub.3, SiO.sub.2, BaTiO.sub.3, Si.sub.3 N.sub.4, etc. The light emitting layer 4 is obtained by electron beam evaporation of a sintered pellet made of ZnS and Mn mixed with each other. At this time, in the sintered pellet, Mn acting as the activator is added, at a composition required for obtaining desired light emitting characteristics, to ZnS acting as the base material and therefore, 0.05-2.5% by weight of Mn is uniformly doped in ZnS. Meanwhile, the second dielectric layer 5 is made of one selected from the group of materials of the first dielectric layer 3 such that the light emitting layer 4 is embedded between the first and second dielectric layers 3 and 5. The back electrode 6 is prepared by employing a resistance wire heating method. The transparent electrode 2 and the back electrode 6 are connected to an AC power source such that a drive voltage is applied to the prior art thin film light emitting elements. The prior art ZnS:Mn type thin film light emitting elements referred to above have such features as light emission at a high brightness upon application of an AC electric field of a few kHz thereto and a long life.
When the prior art ZnS:Mn type thin film light emitting elements are subjected to the drive voltage, an electric field is generated in the light emitting layer 4 so as to excite and accelerate electrons in a conduction band such that the electrons are provided with a large amount of energy. The electrons, in turn, excite luminous centers of Mn through collision therewith, so that the luminous centers of Mn emit light of yellowish orange color when returning to the ground state. In the case where rare earth fluorides, etc. are used for the luminous centers in place of Mn, various luminescent colors such as red, green, blue, white, etc. specific to the respective rare earth fluorides, etc. are obtained.
Referring to FIG. 2, characteristics of relation between luminous brightness and applied voltage (hereinbelow, referred to as "B-V characteristics") of the known thin film light emitting elements of FIG. 1 is shown. It will be readily seen from FIG. 2 that the applied voltage has a threshold value. Namely, when the applied voltage exceeds a threshold voltage Vth, the luminous brightness increases suddenly. When the applied voltage is further raised, the luminous brightness reaches a saturated state. However, this characteristic curve is located at a lower voltage side as shown by the broken line in FIG. 2 immediately after manufacture of the prior art elements. Then, this characteristic curve shifts to a higher voltage side during operation of the prior art elements. In order to obtain a stable characteristic curve, the prior art elements after manufacture thereof are required to be operated for a predetermined time period and then, are driven at the position shown in the solid line in FIG. 2, where the characteristic curve is fixed. By performing this initial operation (referred to as a "stabilizing treatment", hereinbelow), stable electroluminescence corresponding to the applied voltage is obtained.
It is well known as shown in FIG. 3 that the prior art thin film light emitting elements have such a hysteresis characteristic that there exists at any identically applied voltage a difference in value of the luminous brightness between a process for raising the applied voltage and a process for lowering the applied voltage. When light, an electric field, heat, etc. are applied to the known thin film light emitting elements having this hysteresis characteristic, the known thin film light emitting elements are excited to a state of luminous brightness corresponding to the strength of the applied light, electric field, heat, etc. and are maintained at a high luminous brightness even when having been returned to an original state by removing the light, electric field, heat, etc., thereby imparting a so-called memory effect to the known thin film light emitting elements. Consequently, at present, information display units based on the memory effect attract public attention.
More specifically, referring to FIG. 4, there is shown one example of the B-V characteristics of the known thin film light emitting elements. Electrons, which are excited in a conduction band and accelerated by an electric field induced in the light emitting layer in response to application of an AC voltage to the known thin film light emitting elements, obtains a sufficiently large amount of energy so as to act as free electrons. The free electrons are attracted to interfaces of the light emitting layer and accumulated thereon so as to cause internal polarization. At this time, since the free electrons moving at a high velocity excite luminous centers of Mn, etc. directly, the excited luminous centers emit electroluminescent light of yellowish orange color, etc. when returning to the ground state as described earlier. In FIG. 4, when write pulses of a voltage higher than the threshold voltage Vth are applied to the known thin film light emitting elements, the electroluminescent light is set to a state of high luminous brightness in accordance with the characteristic curve. Subsequently, when the applied voltage is lowered to a sustaining voltage Vs for generating sustaining pulses, the internal polarization of a high electric field produced by the write pulses is sustained and the electroluminescent light is maintained at the state of high luminous brightness. Then, when the applied voltage is further lowered to an erasing voltage Ve for generating erasing pulses, the internal polarization sustained in the light emitting layer vanishes suddenly such that the electroluminescent light is set to an erased state. Accordingly, even if the sustaining pulses of the sustaining voltage Vs are again applied to the known thin film light emitting elements in the erased state, it is impossible to obtain the electroluminescent light. When the sustaining voltage Vs for generating the sustaining pulses is selected and proper values are, respectively, assigned to the write and erasing pulses, it becomes possible to achieve the memory effect of the electroluminescence, which is based on the above described hysteresis characteristic. It is to be noted that a potential difference between the characteristic curve at the time of rise the of the applied voltage and that at the time of the drop of the applied voltage is referred to as a memory width Vm. By utilizing the memory effect based on the hysteresis characteristic, it becomes possible, for example, to easily increase the number of lines of electrodes in a display method employing an X-Y matrix type electrode structure, thereby enabling display of high resolution and high density.
It is also known that the above described hysteresis characteristic can be obtained by properly controlling the concentration of the activator (for example, Mn) in the base material (for example, ZnS, ZnSe) of the light emitting layer.
However, the known thin film light emitting elements have such an inconvenience that, since the memory width Vm gradually increases to a saturated state as the known thin film light emitting elements are operated immediately after manufacture thereof, the known thin film light emitting elements are required to be subjected to the time-consuming stabilizing treatment for stabilizing the B-V characteristics, thereby resulting in rise of the production cost.
Furthermore, the prior art thin film light emitting elements have such a disadvantage as operational instability with respect to the hysteresis characteristics.